(MIMO-AMC)
In recent years, there has been a demand in the radio communications field for a technology for transmitting large-volume digital data using a limited frequency band, and transmission scheme using MIMO spatial-multiplexing is known as one method for realizing this.
Also, AMC (Adaptive Modulation and Channel coding) is known as a method for improving transmission efficiency. A method combining MIMO and AMC (hereinafter referred to as MIMO-AMC) has been proposed whereby a modulation scheme and code rate are changed for each transmit antenna so as to adapt to the state of a channel traversed by a signal transmitted from each antenna of a transmitting apparatus. The technology described in Non-patent Document 1 is such a MIMO-AMC technology. The configuration of a transmitting apparatus based on Non-patent Document 1 is shown in FIG. 1. Base station 100 in FIG. 1 illustrates a configuration of a transmitting apparatus based on the technology described in Non-patent Document 1. This configuration is described below taking a case in which the base station transmits information code to a terminal as an example.
Information code is input to base station 100. Information code input to base station 100 is arranged into a parallel sequence from a serial sequence by serial/parallel conversion section 101. Serial/parallel conversion section 101 switches sequences that are output on a timing-by-timing basis, so that information code input at a particular timing is output as sequence #A, and information code input at the next timing is output as sequence #B, for example.
Information code arranged into parallel format is channel-encoded by encoding sections 102 and 103. An error correction code such as a block code, convolutional code, or the like, for example, is used in channel encoding, Encoding sections 102 and 103 can perform channel encoding using a plurality of coding rates, and which coding rate is used for encoding is controlled by adaptive modulation control section 113.
A channel encoded codeword is mapped by mapping sections 104 and 105. The mapping executed here is processing that maps post conversion code to a baseband signal point of a selected modulation scheme. Mapping sections 104 and 105 have mapping patterns of a plurality of modulation schemes, and which modulation scheme is selected is controlled by adaptive modulation control section 113.
Next, frame configuration sections 106 and 107 configure a frame such as shown in FIG. 2. First, frame configuration section 106 inserts a preamble signal in a stream #A frame in time 1. A preamble signal is a signal sequence known to a receiving apparatus, and the receiving apparatus performs synchronization with a received frame by finding a correlation with a preamble signal. An M sequence or the like may be used as a preamble signal, for example. A characteristic of M sequence autocorrelation is that a correlation value when there is no time deviation shows a peak, and a correlation value when there is time deviation shows an extremely low value.
Consequently, the receiving apparatus can find synchronization timing by means of M sequence sliding correlation. Therefore, the receiving apparatus can establish synchronization timing by finding preamble signal sliding correlation value and taking a time at which the correlation value shows a peak value as synchronization timing.
Frame configuration section 107 inserts a null signal in stream #B in time 1. Here, a null signal represents a void signal. In time 2, a pilot signal is inserted in stream #A. This pilot signal is a signal known to the receiving apparatus. The receiving apparatus performs channel estimation based on channel fluctuation undergone by this pilot signal in the course of propagation. A received signal of a receiving apparatus that receives a pilot signal transmitted in time 2 is represented by the following equation.
[1]y=Hp+n  (Equation 1)
where y=[y1 y2]T represents a received signal,
  H  =      [                                        h            11                                                h            12                                                            h            21                                                h            22                                ]  represents a channel matrix,
  p  =            [                                                  p              1                                                          p              2                                          ]        T  represents a pilot signal, and n=[n1 n2]T represents noise. Noise power is assumed to be n2.
Also, “[ ]” are symbols representing a matrix or vector, and T indicates that matrix or vector elements are transposed.
Since the pilot signal in time 2 is p=[p1 0]T, a received signal is as shown by the following equation.
                    [        2        ]                                                                                                                [                                                                                                                              y                          1                                                ⁢                                                                                                        t                            =                            2                                                                                                                                                                                                                            y                          2                                                ⁢                                                                                                        t                            =                            2                                                                                                                                              ]                            =                            ⁢                                                                    [                                                                                                                        h                            11                                                                                                                                h                            12                                                                                                                                                                            h                            21                                                                                                                                h                            22                                                                                                                ]                                    ⁡                                      [                                                                                                                        p                            1                                                                                                                                                0                                                                                      ]                                                  +                                  [                                                                                                              n                          1                                                                                                                                                              n                          2                                                                                                      ]                                                                                                        =                            ⁢                              [                                                                                                                                                          h                            11                                                    ⁢                                                      p                            1                                                                          +                                                  n                          1                                                                                                                                                                                                                              h                            21                                                    ⁢                                                      p                            1                                                                          +                                                  n                          2                                                                                                                    ]                                                                        (                  Equation          ⁢                                          ⁢          2                )            
Here, t represents time. A channel estimation value may be found by the receiving apparatus as shown by the following equation.
                    [        3        ]                                                                                                                [                                                                                                                              h                          ^                                                11                                                                                                                                                                          h                          ^                                                21                                                                                            ]                            =                            ⁢                              [                                                                                                                              y                          1                                                ⁢                                                                                                                                        t                              =                              2                                                                                ⁢                                                      /                                                          p                              1                                                                                                                                                                                                                                                        y                          2                                                ⁢                                                                                                                                        t                              =                              2                                                                                ⁢                                                      /                                                          p                              1                                                                                                                                                                          ]                                                                                        =                            ⁢                              [                                                                                                                              h                          11                                                +                                                                              n                            1                                                    /                                                      p                            1                                                                                                                                                                                                                            h                          21                                                +                                                                              n                            2                                                    /                                                      p                            1                                                                                                                                              ]                                                                        (                  Equation          ⁢                                          ⁢          3                )            
By this means, the receiving apparatus can obtain a channel estimation value for which the SN ratio is p12/n2.
Similarly, frame configuration section 107 inserts a pilot signal in stream #B in time 3. Since the pilot signal in time 3 is p=[0 p2]T, a received signal is as shown by the following equation.
                    [        4        ]                                                                                                                [                                                                                                                              y                          1                                                ⁢                                                                                                        t                            =                            3                                                                                                                                                                                                                            y                          2                                                ⁢                                                                                                        t                            =                            3                                                                                                                                              ]                            =                            ⁢                                                                    [                                                                                                                        h                            11                                                                                                                                h                            12                                                                                                                                                                            h                            21                                                                                                                                h                            22                                                                                                                ]                                    ⁡                                      [                                                                                            0                                                                                                                                                  p                            2                                                                                                                ]                                                  +                                  [                                                                                                              n                          1                                                                                                                                                              n                          2                                                                                                      ]                                                                                                        =                            ⁢                              [                                                                                                                                                          h                            12                                                    ⁢                                                      p                            2                                                                          +                                                  n                          1                                                                                                                                                                                                                              h                            22                                                    ⁢                                                      p                            2                                                                          +                                                  n                          2                                                                                                                    ]                                                                        (                  Equation          ⁢                                          ⁢          4                )            
A channel estimation value may be found by the receiving apparatus as shown by the following equation.
                    [        5        ]                                                                                                                [                                                                                                                              h                          ^                                                12                                                                                                                                                                          h                          ^                                                22                                                                                            ]                            =                            ⁢                              [                                                                                                                              y                          1                                                ⁢                                                                                                                                        t                              =                              3                                                                                ⁢                                                      /                                                          p                              2                                                                                                                                                                                                                                                        y                          2                                                ⁢                                                                                                                                        t                              =                              3                                                                                ⁢                                                      /                                                          p                              2                                                                                                                                                                          ]                                                                                        =                            ⁢                              [                                                                                                                              h                          12                                                +                                                                              n                            1                                                    /                                                      p                            2                                                                                                                                                                                                                            h                          22                                                +                                                                              n                            2                                                    /                                                      p                            2                                                                                                                                              ]                                                                        (                  Equation          ⁢                                          ⁢          5                )            
By this means, the receiving apparatus can obtain a channel estimation value for which the SN ratio is p22/n2.
In time 4, a signal indicating a modulation scheme applied to stream #A is inserted as a signal for notification to the receiving apparatus. In time 5, a signal indicating a modulation scheme applied to stream #B is inserted as a signal for notification to the receiving apparatus. In time 6, a signal for notification of a code rate applied to stream #A is inserted. In time 7, a signal for notification of a code rate applied to stream #B is inserted. In time 8 onward, modulated signals transmitted in streams #A and #1 are inserted. Here, information on modulation schemes and code rates applied to streams #A and #B is obtained from adaptive modulation control section 113.
Frame configuration sections 106 and 107 configure a transmit frame as described above.
Radio section 108 converts a frame configured by frame configuration sections 106 and 107 to a radio signal of a radio frequency band used by the communication system.
Power control sections 109 and 110 convert a radio signal generated by radio section 108 to a radio signal having power equivalent to transmission power. Power conversion here is controlled by adaptive modulation control section 113, and is controlled so that transmission power is the same for streams #A and #B.
Here, encoding sections 102 and 103, mapping sections 104 and 105, frame configuration sections 106 and 107, and power control sections 109 and 110 are controlled by adaptive modulation control section 113. This control is as follows.
First, a stream quality notification frame fed back from the receiving apparatus is input to adaptive modulation control section 113. This stream quality notification frame includes a parameter indicating received stream quality in the receiving apparatus. For example, the SINR (Signal to Interference and Noise power Ratio), received power, or the like of each stream is used as received stream quality.
Adaptive modulation control section 113 obtains information on received stream quality in the receiving apparatus from this stream quality notification frame. Adaptive modulation control section 113 has a table associating stream quality with a modulation scheme, coding rate, and transmission power. Adaptive modulation control section 113 controls adaptive modulation using this table. That is to say, the table contains combinations of modulation scheme, coding rate, and transmission power that improve transmission efficiency according to stream quality.
Using this table, adaptive modulation control section 113 decides a coding rate, modulation scheme, and transmission power corresponding to a received stream quality signal, and controls encoding sections 102 and 103, mapping sections 104 and 105, and power control sections 109 and 110 so as to correspond to the table. Also, adaptive modulation control section 113 controls frame configuration sections 106 and 107 so that stream #A and #B modulation scheme notification signals and coding rate notification signals correspond to a transmitted frame. Antennas 111 and 112 perform spatial-multiplex transmission of transmit signals output from power control sections 109 and 110.
As described above, Non-patent Document 1 discloses a technology that improves transmission efficiency by performing adaptive modulation for each stream that undergoes MIMO spatial-multiplexing.
A method of mapping a channel encoded codeword to a baseband signal is described in Non-patent Document 2. FIG. 3 shows the configuration of a transmitting apparatus that performs mapping according to Non-patent Document 2. In FIG. 3, in which parts corresponding to those in FIG. 1 are assigned the same reference codes as in FIG. 1, as compared with base station 100 in FIG. 1, base station 300 is equipped with source signal point mapping sections 301 and 302 instead of mapping sections 104 and 105. In addition, base station 300 has normalization coefficient multiplication sections 303 and 304.
Source signal point mapping sections 301 and 302 map a channel encoded codeword to a source signal point. Here, a source signal point is defined as a signal point shown in FIG. 4(a). FIG. 4(a) shows QPSK and 16QAM source signal points.
In mapping shown in Non-patent Document 2, a baseband signal point is generated by multiplying a source signal point by a normalization coefficient. This configuration corresponds to normalization coefficient multiplication sections 303 and 304 in base station 300. Here, the normalization coefficients by which QPSK and 16QAM source signal points are multiplied are as shown in FIG. 4(a).
Normalization coefficient multiplication sections 303 and 304 multiply QPSK source signal points by a 1/√2 normalization coefficient, and 16QAM source signal points by a 1/√10 normalization coefficient. Normalization coefficient multiplication sections 303 and 304 generate a baseband signal by performing this normalization coefficient multiplication.
Baseband signal point constellation generated by the above is shown in FIG. 4(b). At the same time, FIG. 4(b) shows pilot-signal signal point placement. Here, mapping sections 104 and 105 in base station 100 perform mapping so that mapping output constitutes the baseband signal points shown in FIG. 4(b).
(MLD)
One technology for MIMO spatial-multiplexed signal detection in a receiving apparatus is MLD (Maximum Likelihood Detection). MLD obtains good characteristics compared with ZF (Zero Forcing), MMSE (Minimum Mean Squared Error), or suchlike spatial-multiplexed signal detection, and has therefore attracted attention as a spatial-multiplexed signal detection technology that enables high-quality reception characteristics to be obtained.
Although spatial-multiplexed signal detection by means of MLD shows excellent characteristics, the arithmetic calculation scale is large, and therefore inventions have appeared that focus on reducing the MLD arithmetic calculation circuit scale. One such method of reducing the MLD arithmetic calculation circuit scale is described in Patent Document 1. A brief description of the MLD arithmetic calculation shown in Patent Document 1 is given below.
First, in the following description, a case in which communication is performed using a two-stream spatial-multiplexing MIMO method will be considered. At this time, a received signal can be represented as shown by the following equation.
[6]y=Hx+n  (Equation 6)
where x=[x1 x2]T represents a transmit signal.
Here, the square Euclidean distance of a received signal is found as shown by the following equation.
                    [        7        ]                                                                                                E              =                            ⁢                                                                                      y                    -                    Hs                                                                    2                                                                                        =                            ⁢                                                                                                            y                      1                                        -                                                                  (                                                                                                            h                              11                                                        ⁢                                                          s                              a                                                                                +                                                                                    h                              12                                                        ⁢                                                          s                              b                                                                                                      )                                            2                                                                                        +                                                                                                                        y                        2                                            -                                              (                                                                                                            h                              12                                                        ⁢                                                          s                              a                                                                                +                                                                                    h                              22                                                        ⁢                                                          s                              b                                                                                                      )                                                                                                  2                                                                                        (                  Equation          ⁢                                          ⁢          7                )            
As y is a received signal, it can be found by the receiving apparatus. H is estimated by the receiving apparatus using a pilot signal. The estimation method is as shown in Equation (3) and Equation (5). Also, s is a vector representing transmit symbols that can be used by the transmitting apparatus, and s=[sa sb]T. Hereinafter, sa and sb are referred to as candidate signal points. In MLD, a combination of candidate signal points sa and sb for which Equation (7) is a minimum is taken as a detection result.
The invention disclosed in Patent Document 1 shows in particular that it is possible for the circuit scale to be reduced in spatial-multiplexed signal detection even if the M-ary modulation value is large. The technology disclosed in Patent Document 1 generates a replica signal corresponding to each transmit signal.
Replica signals here are h11sa+h12sb and h21sa+h22sb shown in Equation (7).
In the technology disclosed in Patent Document 1, terms h11sa+h12sb and h21sa+h22sb configuring replica signals are stored. Furthermore, addition of these replica signals is performed taking account of a combination of terms configuring the stored replica signals. It is shown that the number of complex multipliers necessary for replica signal generation can be reduced by this means.    Patent Document 1: Japanese Patent Application Laid-Open No. 2006-196989    Non-patent Document 1: 3GPP TSG RAN WG1, TSG-R1-010879, Lucent Technologies, “Increasing MIMO throughput with per-antenna rate control”    Non-patent Document 2: ISO/IEC and redesignated as ISO/IEC 8802-111:1999/Amd 1:2000, “Supplement to IEEE Standard for information technology—Telecommunications and information exchange between systems—Local and metropolitan area networks—Specific requirements. Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications High-speed Physical Layer in the 5 GHz Band,” IEEE Std 802.11a-1999    Non-patent Document 3: “Linear Algebra and its Applications,” G. Strang, Sangyo Tosho Publishing Co., Ltd.